Auto-calibration of pressure transducer offset

ABSTRACT

A flow generator having use in the provision of CPAP or assisted ventilation treatment is disclosed The flow generator has a turbine for the supply of breathable gas which is driven by an electric motor and an associated motor power supply. The power supply in turn is controlled by a controller. A pressure sensing port measures delivery pressure that is passed to a pressure transducer, the output signal of which is passed to the motor controller. The zero offset of the pressure transducer is automatically calibrated on occurrence of the conditions where the turbine is not operating and no pressure activity is sensed by the transducer. The turbine not operating condition can be determined from Hall-effect sensors integral to the motor.

This application is a divisional of application Ser. No. 08/894,216filed Feb. 3, 1998, now U.S. Pat. No. 6,237,592, which is a 371 ofPCT/AU96/00413 filed Jul. 3, 1996, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

This invention relates to the auto-calibration of pressure transducers.In one preferred form, it relates to pressure transducers for use inapparatus for the provision of Continuous Positive Airway Pressure(CPAP) treatment to patients suffering from obstructive Sleep Apnea(OSA) and for use in apparatus for ventilating assistance.

BACKGROUND OF THE INVENTION

CPAP is a well known treatment for the temporary relief of conditionsincluding obstructive sleep apnea (OSA) and snoring. By this technique,air (or breathable gas) at a pressure elevated above atmosphericpressure is continuously supplied to the entrance of a patient's airway(by the nasal and/or oral route) by means of known arrangements of masksor nasal prongs. The elevated air pressure acts as a pneumatic splint ofthe patient's airway in the vicinity of the oro- and hypo-pharynx,reducing or eliminating the occurrences of apneas or hypopneas duringsleep. A bi-level CPAP device, as opposed to a constant treatment levelCPAP device, delivers two distinct pressures during the patient'srespiratory cycle—a relatively lower pressure during exhalation and arelatively higher pressure during inhalation. In another form, anautomatically adjusting CPAP device may operate to provide a relativelylow background pressure which increases to a therapeutic pressure on aneeds basis, and preferably at a time to prevent the onset of an apnea.

The term “CPAP” used herein thus is to be understood as includingconstant, bi-level or adjusting forms of continuous positive airwaypressure.

Common to all forms of CPAP apparatus is a nose, mouth or face maskwhich is fitted to a patient and connected to a flow generator via aflexible air delivery tube/conduit. The flow generator includes anelectric motor driving a turbine to provide a supply of air orbreathable gas for the administration of CPAP treatment during sleep.The range of positive air pressures supplied at the entrance to apatient's airway typically is in the range 2-20 cm H₂O. In the pressureregulation control of the flow generator it is usual to have acontinuous measure of mask or flow generator delivery pressure, commonlyachieved by locating a pressure sensing port at the mask or proximatethe flow generator outlet.

In the clinical assessment of the severity of a patient's OSA or upperairway syndrome condition, it is desired to identify the minimumpossible CPAP treatment pressure that will alleviate the occurrence ofpartial or complete apneas during sleep. This is for the reason that thepatient is required to expend respiratory effort in expiration againstthe positive airway pressure, hence it is preferable to minimise thework that must be done to ensure quality of sleep, and as followsadminister only the minimal necessary CPAP treatment pressure. In thisregard, it is important that the pressure transducer being used tomeasure the CPAP treatment pressure in control of the flow generator hassatisfactory electro-mechanical characteristics so that the set-pointCPAP treatment pressure does not vary significantly. It is known that areduction of CPAP treatment pressure of as little as 1 cm H₂O cannullify the therapeutic effect and result in a patient experiencingapneas during sleep.

There is, not unexpectantly, a direct correlation between theelectro-mechanical performance of pressure transducers and price, hencethe need for accurate pressure measurement is antagonistic towards theneed to be able to manufacture CPAP apparatus at a cost that isacceptable to the marketplace. Commercially available pressuretransducers, that are not extraordinarily expensive, operate in a smallpart of their pressure dynamic range in CPAP applications, meaning thatthere can be a 5-10% drift in the measured value with time due wholly toa pressure transducer operating in a ‘stretched’ region of operation.Such a variation translates to a variation in CPAP treatment pressure ofabout 1-2 cm H₂O. There further is market pressure for CPAP treatment tobe determined to within an accuracy as low as 0.1 cm H₂O.

It is therefore one preferred object of the invention to be able toavoid the need to incorporate expensive pressure transducers in CPAPapparatus and yet still maintain accurate monitoring of, and controlover, CPAP treatment pressure.

A similar consideration applies for ventilators or apparatus forassisted ventilation that provide breathable gas to a patient at acontrolled pressure. The gas is delivered to the patient, in the case ofa ventilator, by way of a mask or an endotracheal tube. Patients withlung disease, neuromuscular disease, chest wall disease, orabnormalities of respiratory control may require ventilatory assistance.This is because they have various combinations of elevated airwayresistance, stiff lungs and chest wall, ineffective respiratory muscles,or insufficient neural activation of the respiratory muscles. The needfor ventilatory assistance is particularly common during sleep. Pressurecontrolled, time triggered ventilators, for example, deliver arelatively high inspiratory pressure (IPAP) for a fixed period of time(TI), and a relatively low expiratory pressure (EPAP) for another fixedperiod of time (TE). The cycle is then repeated indefinitely.

Pressure transducers typically are factory calibrated before delivery,to establish a zero pressure value (with respect to CPAP treatmentpressure that is relative to atmospheric pressure) in terms of thetransducer's offset or bias. The “zero offset value” thus corresponds toatmospheric pressure. Even so, due to the inherent variations in thetransduced pressure, and due to aging of the transducer and itstemperature dependency, the preset offset value can vary by theequivalent of ±1 cm H₂O leading to measurement error. This means thatthe patient must periodically return the CPAP apparatus to themanufacturer or servicer for re-calibration, else perform are-calibration procedure themself, possibly requiring venting of thetransducer to atmospheric pressure. It is therefore another preferredobject of the invention to provide for auto-calibration of pressuretransducer offset.

For convenience any reference to a “mask” hereafter is to be understoodas including nasal, oral or face masks, and nasal prongs.

SUMMARY OF THE INVENTION

The present invention is directed to methods and apparatus whereby oneor more of the foregoing problems can be overcome or at leastameliorated.

Therefore, in a broad form the invention discloses a method forauto-calibration of the offset of a pressure transducer for use in CPAPor pressure regulated ventilation apparatus, the CPAP apparatuscomprising a flow generator operable to supply breathable gas to adelivery tube in turn connected to a patient mask, and the pressuretransducer measuring delivery pressure in the mask, delivery tube orflow generator, the method comprising the steps of:

determining whether the flow generator is operating;

determining whether there is no pressure activity sensed by thetransducer; and

if both determinations are satisfied, accepting the output of thetransducer as a calibrated pressure offset value representative ofatmospheric pressure.

The invention further discloses a method for auto-calibration of theoffset of a pressure transducer for use in CPAP or pressure regulatedventilation apparatus, the apparatus comprising a flow generatoroperable to supply breathable gas to a delivery tube in turn connectedto a patient mask, and the pressure transducer measuring deliverypressure in the mask, delivery tube or flow generator, the methodcomprising the steps of:

determining whether no pressure activity is continuously sensed by thetransducer over a predetermined period of time, and if so accepting theoutput of the transducer as a calibrated pressure offset valuerepresentative of atmospheric pressure.

The invention yet further discloses a flow generator for the supply ofbreathable gas comprising an electric motor driving a turbine, controlcircuitry, a pressure transducer to sense delivery pressure at or remotefrom said flow generator and whose electrical output is connected tosaid control circuitry, and sensing means connected to said controlcircuitry, and wherein said control circuitry is operable to determinefrom said sensing means whether the electric motor or the turbine areoperating and from the pressure transducer whether there is no pressureactivity, and if both determinations are satisfied, to accept the outputof the transducer as a calibrated pressure offset value representativeof atmospheric pressure.

The invention yet further discloses a flow generator for the supply ofbreathable gas comprising an electric motor driving a turbine, controlcircuitry, and a pressure transducer to sense delivery pressure at orremote from said flow generator and whose electrical output is connectedto said control circuitry, and wherein said control circuitry isoperable to determine from the pressure transducer whether there is nopressure activity continuously over a predetermined period of time, andif so, to accept the output of the transducer as a calibrated pressureoffset value representative of atmospheric pressure.

An auto-calibration thus can be performed in the sense that there is nouser/patient involvement nor manual activation, rather performance bythe flow generator autonomously. If either determination is not met anauto-calibration is not performed.

The pressure offset value is applied to all subsequent pressuremeasurements to determine treatment pressure. Preferably, the twodeterminations are made over a plurality of successive instances andmust both be satisfied on each instance before accepting the updatedoffset value.

In a preferred form, the current pressure value is compared against apreceding pressure value, and if differing less than a predeterminedthreshold then there is pressure inactivity. The preceding pressurevalue can be a running or moving average of such values.

The electric motor operation sensing means can be Hall-effect sensorsintegral of, or mounted to the electric motor. The flow generator canfurther comprise power supply means being controlled by the controlcircuitry and having control of the rotational speed of the electricmotor.

The invention further discloses a flow generator as described, and adelivery tube coupled thereto and to a patient mask. The mask can be anose, mouth or face mask. The pressure sensing port can be located atthe turbine exit, a point along the tube or at the mask.

In one preferred form, the invention can be said to involve methods andapparatus for providing auto-calibration of pressure transducer offsetthat is implemented by continuously monitoring the flow generatorelectric motor to determine whether it is running and monitoring thepressure transducer for respiratory activity. If the motor is notrunning, and no pressure activity is detected, then the pressuremeasured by the transducer is determined to be atmospheric pressure, andso the electrical output from the transducer represents the zeropressure offset. The offset value at this point in time is stored to besubtracted from any subsequent pressure measurement values to determinetreatment pressure.

In an alternate embodiment, auto-calibration of pressure transduceroffset is implemented by determining whether there is no pressureactivity sensed by the pressure transducer over a continuous period thatis long compared with physiological events such as respiration andapnea. This single determination thus subsumes the separatedeterminations of motor operation and respiratory activity.

The zero offset can be updated whenever the opportunity arises, thustaking into account effects due to transducer ageing and temperatureeffects. In this way, the magnitude of a transducer's offset error canbe determined automatically without the need for additional hardwareelements, such as a solenoid-operated valve venting the transducer toatmospheric. There also is no need for any user intervention in theperiodic recalibration. This leads to a reduction in the cost of thehardware components of a CPAP or pressure regulated ventilationapparatus, and to a reduced manufacturing unit cost due to a reductionin labour required, for reason of there being no need to manuallycalibrate a transducer at the factory in advance of shipment, incombination resulting in reduced cost of the apparatus to the patient.

The improved pressure measurement accuracy gained also has therapeuticbenefit, in that the CPAP or ventilation treatment will remain effectivefor a patient, in that the clinically-determined delivery pressure ismaintained with accuracy.

DESCRIPTION OF THE DRAWINGS

An embodiment of the invention now will be described with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic block diagram of CPAP apparatus as used for thetreatment of OSA;

FIG. 2 is a schematic block diagram representative of hardwarecomponents constituting an embodiment of the invention;

FIG. 3 is a schematic block diagram of representative computationalsteps in performance of the embodiment;

FIG. 4 is a schematic block diagram representative of hardwarecomponents constituting another embodiment;

FIG. 5 is a schematic block diagram of representative computationalsteps in performance of another embodiment; and

FIG. 6 shows a typical pressure waveform during calibration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS AND BEST MODE

The embodiment to be described relates to CPAP apparatus, however it isto be understood that other embodiments are equally applicable in thefield of pressure regulated ventilators.

Referring then to FIG. 1, the CPAP apparatus comprises a flow generator10 coupled by a flexible delivery tube or conduit 12, in this case, to anose mask 14 worn by a patient 16. The flow generator 10 broadlycomprises an electric motor 18 that is powered by a motor power supply20. In turn, the electric motor 18 has mechanical coupling with aturbine 22 that outputs either air or breathable gas at a pressureelevated above atmospheric pressure to the delivery tube 12. The outputdelivery pressure from the turbine 22 is governed by the rotationalspeed of the electric motor 18. The speed thus is the “controlledvariable” relative to the desired CPAP treatment pressure. The motor 18speed is controlled by the motor controller 23 which effects changes inmotor speed by means of a control signal on control line 24 provided tothe motor power supply 20. Accordingly, motor speed is controlled bymeans of varying the motor power supply 20, typically in output voltageand/or duty cycle.

The motor controller 23 receives an electrical signal on control line 26in this case representative of delivery pressure from the turbine 22 asmeasured by the pressure transducer 28, which is connected via a sensingline 29 to a sensing port 27 proximate to the turbine outlet. In analternative form, the pressure transducer 28 can be connected via asensing line 29′ to a sensing port 27′ located at the nose mask 14.

In the event of the transducer 28 being located at the turbine 22outlet, it is necessary for the motor controller to be able tocompensate for pressure losses (as a function of flow) along thedelivery tube 12, since ultimately it is the pressure at the entrance tothe airway that is to be monitored and controlled to ensure effectiveCPAP treatment. This compensation can be performed by empiricalmeasurement or by a knowledge of the flow vs pressure characteristic ofthe delivery tube 12.

In one preferred form, the turbine 22 can be a PAPST™ ECA 27-11brushless DC motor. Being a DC motor, its speed is directly proportionalto the armature voltage. The particular motor described has integralHall-effect sensors, thus providing a measure of motor angularrotational speed, that signal being output from the motor 18 to themotor controller 22 on control line 30.

The pressure transducer 28 can be such as a Motorola™ MPX 2010DP type.The motor controller 23 can be implemented by any commercially availablemicroprocessor, although one preferred form is the 8-bit Motorola™MC68HC805B6 micro-controller.

When the motor 18 is not running, hence the turbine 22 is not rotatingto produce pressurised air or breathable gas, if the nose mask 14 is notbeing worn then the air pressure measured by the transducer 28 will beatmospheric pressure. The transducer 28 will have an electrical outputin response only to atmospheric pressure. The measurement obtained ofatmospheric pressure represents the “zero offset” value, and hence themotor controller 23 must be calibrated to take into account this zerooffset so accurate measurements can be performed. That “zero offset”output must be subtracted from a pressure value measured with theturbine 22 rotating in order to obtain a measurement of CPAP deliverypressure.

As previously discussed, the electrical performance of pressuretransducers varies over time, and whilst the linearity may remainessentially constant, the zero offset can vary by 5-10% The presentembodiment operates to auto-calibrate the transducer in terms of the“zero offset” at available times, viz., occasions when the flowgenerator 10 is not being operated and the mask 14 is not being worn (orthere is no respiration) by the patient 16.

In FIG. 2 the elements common with FIG. 1 are shown using like referencenumerals. The electric motor 18 is represented by the component motor 40and integral Hall-effect (speed) sensors 42. Within the dashed boxrepresenting the motor controller 23, only some number of the logicelements constituting the controller have been shown. These logicelements are the ones involved in generation of a “Transducer Offset”signal that equates to the “zero offset” value of the transducer andthus atmospheric pressure.

The output signal from the speed sensors 42 is passed on a control line32 to a motor activity detector 50, and the signal representing themeasured pressure appears on another control line 26 to be passed to apressure activity detector 52. Both the motor activity detector 50 andthe pressure activity detector 52 pass logic signals on respective lines58, 60 to a time-out checker 54, the output from which is passed on aline 62 to a transducer offset capture element 56 that also receives theoutput of the pressure transducer 28. These two signals are processed togenerate the “Transducer Offset” value to be passed to other processingelements of the motor controller 23.

Reference now will be made to the flow diagram of FIG. 3 and the logicand computational steps therein that further describe the presentembodiment. These steps are performed by the motor controller 23 in theform of either a stored computer program in machine readable form ordiscrete logic elements. Step 60 determines whether a warm-up period haselapsed to allow the flow generator 10 to reach a normal operatingtemperature. A typical minimum warm-up time is 15 minutes. If this stepis satisfied, the “pressure activity counter” is reset (i.e. to zero) instep 62, followed by step 64 that determines whether the motor 40 isoperating/running. This step is performed through use of the Hall-effectsensors 42 and the motor activity detector 50. If the motor is running,it is not possible to perform the auto-calibration. If the motor is notrunning, step 66 determines whether any pressure activity is sensed.This is achieved by way of the pressure transducer 28 and the pressureactivity detector 52. If there is pressure activity then again theauto-calibration cannot be performed. Pressure activity can be definedas an absolute difference between two consecutively measured pressuresamples exceeding a predetermined threshold. In this regard theimmediately preceding value is stored and compared with the currentvalue. The difference is compared with a predetermined threshold,typically set to ±0.5% (approximately equivalent to 1 count in a rangeof 185 counts). If the threshold is not exceeded, it is determined thereis no pressure activity.

If no pressure activity is sensed, then in step 68 the “pressureinactivity counter” is incremented. In step 68 the value of the pressureinactivity counter is compared against a “pressure inactive time-out”value (viz., by the time-out checker 54) that is implemented to ensurethat there is a minimum duration of no motor activity and no pressureactivity before auto-calibration can take place. Thus the tests of steps64 and 66 must be satisfied more than once in the looped-manner shownbefore the test of step 70 will be satisfied and, as indicated in step72, the current pressure transducer output is captured and utilised asan auto-calibration of the Transducer Offset.

The pressure inactive time-out value preferably may be a number ofinteractions equivalent to, say, a two second duration.

The absence of pressure activity corresponds to the absence ofrespiration, which, in most every case, will be due to the mask 14 notbeing worn. The only practical instance of there being a lack ofpressure activity with the mask being worn is if the patient has ceasedbreathing, whether that is a consequence of occurrence of an apnea orotherwise. In any event, it would be even rarer for a patient to bewearing a mask during sleep without the electric motor 40 operating.

The “motor not running” condition and the “mask not being worn”condition will tend to occur when the flow generator 10 is first turnedon, or may arise from the patient stopping the flow generator, or theflow generator automatically stopping itself in the presence of sensing‘mask off’ (whether that be intentional or otherwise).

By the methodology described, the Transducer Offset can be updated everytwo seconds (for example) when the motor 40 is not operating and themask is not being worn, in that these conditions are satisfied manytimes over before the time-out period elapses, but only the pressurevalue immediately preceding the end of the time out period is capturedas the updated Transducer Offset.

In other embodiments it may be preferred to utilise a running or movingaverage of previous (say, at least five) pressure samples compared withthe current pressure sample to determine whether there is pressureinactivity. Such averaging functions can be achieved by the introductionof appropriate software or hardware filters.

In a further embodiment, it may be chosen to base the auto-calibratingTransducer Offset value not upon the instantaneous (atmospheric) sensedpressure, but upon some averaged representation of sensed pressure overa contemporaneous historical period.

In a yet further embodiment, it may also be chosen to separately storethe transducer offset measured prior to satisfying the warm-up conditionin step 60 so that both a “cold-offset” and a “warm-offset” are stored.

An alternative embodiment is shown with reference to FIGS. 4 and 5. Inthis embodiment the determination of whether the flow generator 10 is ina condition such that auto-calibration can take place occurs on thebasis only of the pressure measured by the pressure transducer 28. Thatis, a determination of the CPAP mask not being worn and the turbine 22not operating can be arrived at based only on the pressure measurement.

The steps shown in FIG. 5 are common with the steps previously shown inFIG. 3, but for omission of the “motor running” step 64. The otherdifference occurs in relation to step 70, inasmuch as the “time out”value is set to be long with respect to any physiological event. Typicalphysiological events are respiration and apneas. A time out period ofbetween 2-5 minutes has been determined to be satisfactory.

In the event that a patient mask is not being worn and the electricmotor 40 is operating to cause rotation of the turbine 22, then forreasons of the flow of air past the pressure transducer 28 to bedischarged from the mask into free space, it is the case thatfluctuations or perturbations in pressure will be sensed by the pressuretransducer 28 to even to fluid dynamics effects in the physical vicinityof the pressure transducer 28. A minimum threshold of 0.1 cm H₂O can beutilised to discriminate between pressure activity and pressureinactivity.

FIG. 6 shows a waveform of pressure versus time for the embodiment ofFIGS. 2 and 3, where the time axis is in 10 seconds per divisionincrements. The waveform represents the signal measured by the pressuretransducer 28 with the internal pressure offset applied to providerelative treatment pressure. Time interval A represents the situation ofthe flow generator 10 being turned off, in which case the measuredpressure is 0 cm H₂O relative to atmospheric pressure. Time interval Brepresents the flow generator stepping up to the minimum CPAP pressureof 4 cm H₂O then ramping to the target treatment pressure of 7 cm H₂O.Time interval C represents continuous operation at the desired treatmentpressure. At the end of time interval C, an artificial offset error isintroduced so that the actual pressure generated is 10 cm H₂O whereasthe flow generator believes the delivery pressure is 7 cm H₂O. Thus a 3cm H₂O error in the transducer offset has been introduced. Time intervalD represents the continuing operational period with the error intransducer offset. At the end of interval D the flow generator is turnedoff, and in interval E, the pressure reduces to 0 cm H₂O.

In time interval F two calibration operations are performed. Immediatelyat the commencement of the interval (upon turning on of the flowgenerator), the pressure transducer offset value is recalibrated giventhat the two conditions of the flow generator not operating and therebeing no pressure activity are satisfied. What follows is a period inwhich the flow generator operates at approximately 15 cm H₂O, this beinga gain calibration that is not directly applicable to the presentinvention. After the gain calibration, the flow generator again turnsoff, with the pressure returning to the 0 cm H₂O level. Time interval Grepresents a ramping-up to resumption of CPAP treatment in interval H atthe now correct delivery pressure level of 7 cm H₂O.

What is claimed is:
 1. A method for auto-calibration of the offset of apressure transducer for use in CPAP or pressure regulated ventilationapparatus, the apparatus comprising a flow generator operable to supplybreathable gas to a delivery tube in turn connected to a patient mask,and the pressure transducer measuring delivery pressure in the mask,delivery tube or flow generator, the method comprising: determiningwhether the flow generator is not operating; determining whether thereis no pressure activity sensed by the transducer; and if bothdeterminations are satisfied, accepting the output of the transducer asa calibrated pressure offset value representative of atmosphericpressure.
 2. A method as claimed in claim 1, whereby said no pressureactivity is determined by determining whether there is no absolutedifference between two consecutive measurements of pressure by thepressure transducer.
 3. A method as claimed in claim 2, whereby said noabsolute difference between two consecutive measurements of pressure isdetermined by comparing a stored immediately preceding measured pressurevalue with a current pressure value, comparing the difference against athreshold, and if the threshold is not exceeded, there is no pressureactivity.
 4. A method as claimed in any one of claims 1 to 3, wherebysaid flow generator not operating is determined by detecting near zerorotational speed of the flow generator.
 5. A method as claimed in anyone of claims 1 to 3, whereby said flow generator not operating isdetermined by there being no sensed pressure activity over a continuousperiod of time that is long compared with any physiological event.
 6. Amethod for auto-calibration of the offset of a pressure transducer foruse in CPAP or pressure regulated ventilation apparatus, the apparatuscomprising a flow generator operable to supply breathable gas to adelivery tube in turn connected to a patient mask, and the pressuretransducer measuring delivery pressure in the mask, delivery tube orflow generator, the method comprising: determining whether no pressureactivity is continuously sensed by the transducer over a predeterminedperiod of time, and if so accepting the output of the transducer as acalibrated pressure offset value representative of atmospheric pressure.7. A method as claimed in claim 6, whereby the period of time is longcompared with any physiological event.